It is a common practise to employ in driving such elements as drills, casings and the like, elements which are so constructed that before one element is driven completely into the ground, another element of the same type can be threaded to the exposed end thereof and then driven with other elements added in the same manner until a so-called string is established penetrating the earth to a wanted depth.
It is also a common requirement that such elements, particularly drills and their extensions be withdrawn from the ground with the members of a string disconnected therefrom when each is completely exposed.